For determining a physical measured quantity, the most various types of sensors are used, some of which are based on quite different physical measurement principles. For instance, the level of a product in a container is ascertained by way of the transit time of ultrasound waves or electromagnetic waves and in particular microwaves that are reflected from the surface of the product. When microwaves are used, they are either projected freely into the container in the direction of the surface of the product, or the microwaves are passed into the container along a conductive element.
Moreover, capacitive and radiometric measurement methods are used to measure fill levels.
For limit state detection, the resonant frequency of an oscillator bar or an oscillatable structure that comprises a plurality of oscillator bars is preferably evaluated. In this measuring method, the effect that the resonant frequency differs depending on whether the oscillator bars execute their oscillations freely or in contact with the product is utilized.
No matter how different in structure the various measuring devices are for determining the fill level or some other physical quantity, they have one thing in common: They require energy. In the normal situation, this energy is supplied to them via electrical cords. The disadvantage of all the known measuring devices is now considered to be that the cabling required for supplying energy requires a relatively complicated and expensive installation. The cables themselves are moreover naturally relatively expensive.
It is indeed also known to position solar cells for supplying energy in the vicinity of the measuring devices, but using solar cells is limited to only a few exceptional cases, for well known reasons; solar cells can function only when they are exposed at least intermittently to direct sunlight.